The invention relates to a process for preparing cellular polyurethane molded parts with solid surfaces, in which a combination of CFC blowing agents and of HCFC blowing agents is not used. Nevertheless, this process results in molded items having the same shrinkage as molded parts prepared using the CFC and/or HCFC blowing agents, at the same hardness.
To prepare polyurethane foams, apart from water, fluorochlorocarbons (CFCs) or hydrofluorochlorocarbons (HCFCs) in particular have previously been used as blowing agents. The HCFC type blowing agents have previously been virtually exclusively used for the preparation of semi-rigid molded parts/articles from polyurethane foam with a solid surface, such as, e.g. for covering steering wheels or as soles for shoes. The semi-rigid foams produced using these blowing agents shrink by about 1.5%, which must be taken into account when constructing the mold. The molds used in practice have been designed correspondingly larger due to this approximately constant value for shrinkage of 1.5%, in order to achieve the actual dimensions required after the predicted shrinkage has occurred.
Due to the known ecological problems connected with the halogen-containing blowing agents mentioned, there is a great deal of interest in new kinds of reactive systems which react to give semi-rigid polyurethane foams with solid surfaces and which do not contain halogen-containing blowing agents. It is desired that these foams with solid surfaces exhibit the advantages which are normally associated with the use of these blowing agents, without being tainted by the disadvantages, especially from the ecological point of view. The blowing agents in these new kinds of reactive systems are intended not only to enable the production of molding foams with solid surfaces, but also to result in molded articles whose shrinkage at the same hardness corresponds to the shrinkage of polyurethane foams prepared using the halogen-containing blowing agents. In fact, the molds which have been used up to the present time can only continue to be used if this requirement is met.
The use of water as the only blowing agent does not solve the problems mentioned above. In particular, this is because (i) the decrease in pressure in the foam takes place much more slowly using water than when using known halogen-containing blowing agents, so that at the conventional short molding times used for mass production, molded articles are produced which tend to crack easily, (ii) the elasticity of the resulting molded article (especially for soles of shoes) is not sufficient for the requirements met in practice, and, especially, (iii) the resulting molded articles have a shrinkage of only 0.5% so that the molds previously used would become useless.
The equally obvious idea of using hydrocarbons such as, for example, isomeric pentanes or cyclopentane, has, in particular, the problem of the high flammability of these substances.
It has now been found that specific carbamates, of the type described in more detail below, represent blowing agents which correspond to all the above mentioned prerequisites. In particular, these enable the production of semi-rigid polyurethane foams with solid surfaces, whose shrinkage at comparable hardness corresponds to the shrinkage of the foams previously produced using halogen-containing blowing agents.
Although EP 0,121,850 already describes, inter alia, the use of carbamates of the now recommended type as blowing agents for polyurethane foams. However, it is clear from the working examples that it is preferred that these type of blowing agents be used in combination with other blowing agents. The prior publication, however, does not disclose or suggest that carbamates also enable the production of polyurethane foams with solid surfaces, or the molding foams produced in this way have a shrinkage behavior which corresponds to that of the molding foams in accordance with the prior art as discussed above.